Pulse width modulation (PWM) is a conventional technology used for controlling power converters to achieve output power, voltage, and current regulation. Conventional flyback power converters include a power stage for delivering electrical power from a power source to a load, a switch in the power stage that electrically couples or decouples the load to the power source, and a switch controller coupled to the switch for controlling the on-time and off time for the switch. The on-time (TON) and off-time (TOFF) for the switch can be modified by the controller based on a feedback signal representing output power, voltage, or current. The energy is stored in the transformer core gap when the switch is on, and is transferred to the load circuit when the switch is off. Regulation can be accomplished by, among other things, measuring the output power, voltage, or current, and feeding an indicating signal back to the primary side controller, which can modify the TON-time and TOFF-time of the switch accordingly to effectively regulate the output power, voltage, or current. The switching cycle TS is the sum of the on-time TON and off-time TOFF.
In power supply designs, it is necessary to regulate power, voltage, or current depending upon the application. The present invention is related to the regulation of current in a mode called “Constant Current”, or (CC) mode.
One conventional power supply system involves a flyback converter that senses the output voltage directly from the secondary side of the transformer. This is called secondary sensing. FIG. 1 is an illustration of such a conventional secondary side sensing circuit.
FIG. 1 illustrates a traditional flyback power supply with secondary sensing. It is configured to regulate both voltage in constant voltage (CV) mode and current in constant current (CC) mode. The PWM controller 100 is powered by Vcc which can be derived a number of different ways from the power supply. When the PWM controller begins operation, it outputs a PWM stream to MOSFET 120, which turns on the primary current of transformer 110. As the flyback operation takes place, energy is transferred from the primary side to the secondary side of the transformer during each cycle that over time constitutes an output power to be dissipated over the Load 160. As the output voltage exceeds the sum of the zener diode 140 voltage and the drop across the forward biased diode, which is part of the opto-coupler 150, the opto-coupler diode conducts, and turns on the NPN photo-transistor that is part of the same opto-coupler 150 integrated circuit. When the transistor is turned on, this draws current that causes the controller to reduce the TON-time. In addition, there is a current sense resistor 170 that will develop a voltage drop across the base-emitter junction of transistor 130. When the load resistance 160 is decreased such that the power supply delivers the maximum current, the transistor 130 bypasses the zener diode 140, and causes current regulation.
There are at least two significant drawbacks in cost to this solution. First, it requires the external circuit consisting of the transistor 130 and the current sense resistor 170 to aid the current regulation. The second drawback is the wasted power dissipated by the sense resistor, which reduces the overall system efficiency.
FIG. 2 is an illustration of the ideal VI curve demonstrating the desired performance where the power supply controller transitions from a Constant Voltage (CV) mode to a Constant Current (CC) mode. The shape of this curve is ideally unchanged as a function of what the VIN value is. In fact, this VIN comes from an alternating current (AC) source through a rectifier bridge and a minimal bulk capacitance.
FIG. 3 is an illustration of a conventional system. It is a primary side feedback method where the current regulation takes advantage of knowledge of the input AC voltage and the output voltage. It is the subject of U.S. Pat. No. 6,972,969 that is incorporated by reference herein in its entirety (the Shteynberg patent).
With reference to FIG. 3, a rectifier, e.g., bridge diode (BR1), receives an AC signal that typically has a voltage that ranges from 90V to 264V and outputs a rectified signal. Capacitor C1 acts as a high frequency filter for the output of the BR1 that is coupled to a primary side winding of a transformer T1. Transformer T1 includes a primary and auxiliary winding on the primary side and a secondary winding on the secondary (output) side. In this embodiment, conventional circuitry, e.g., rectifying diodes D2, D3 and filter C3, R1, R can be used to sense the voltage (VSENSE) on the auxiliary winding (primary side). The direct current (DC) input voltage (VINDC) is identified by using, for example, sense resistor R3 in combination with a low frequency filter (C2/R4). The secondary winding is coupled to an output rectifier diode D1 and output filter C0.
In a flyback type power converter that operates in discontinuous conduction mode, the output power Po can be expressed as:
                              P          o                =                                            Vin              2                                      2              ⁢                              L                M                                              ×                                    t              on              2                                      T              S                                ×          η                                    (        1        )                                          I          o                =                                            P              o                                      V              o                                =                                                    Vin                2                                            2                ⁢                                  L                  M                                                      ⁢                          k                              V                o                                      ⁢            η                                              (        2        )            
Where η is the power efficiency (Po/Pin), and
                    k        =                              t            on            2                                T            S                                              (        3        )            
For a given line voltage, the output power is directly proportional to k, which is the ratio of the square of the TON-time to the switching period TS. This means that k is substantially a constant for a particular output voltage. Hence, the output current can be limited based upon the value of the output voltage. The current can be controlled at a constant level based upon the sensed output voltage. The on-time TON and switching period TS are generated by the pulse generator 330. A conventional analog-to-digital (A/D) converter 304 generates the digital feedback voltage signal VFB from the VSENSE signal. The feedback voltage signal is directly proportional to the output voltage under any condition cycle by cycle.
The VFB is sent to the input of a conventional digital error amplifier 306, which generates an error feedback signal VCM. In one embodiment of the present invention, the VCM signal is the proportional-integral (PI) function of the normal (nominal) feedback voltage level VFB—NOM and the feedback voltage which represents the output voltage.
When the output current is increased, the feedback voltage VFB is decreased corresponding to the drop of output voltage. This results in an increase in VCM. The error voltage signal VCM is received by the pulse generator 530 as the control signal VC and is used by the pulse generator 530 to control the on-time (TON) and the switch period TS, to achieve a constant k. Thereafter, the on-time can be increased to deliver more power to output until the output voltage is within the tolerance level. The deviation of the on-time Δton is inversely proportional to the deviation of feedback voltage ΔVFB. Accordingly, when the output current is less than the current limit (as represented by VCM) this loop is a negative feedback loop.
The multiplexor is controlled by the current limit block 320 that sets the control voltage signal VC equal to the error voltage signal VCM when the value of the error voltage signal corresponds to the output current being less than the preset voltage signal VCT that corresponds to the limited output current ILIM. Otherwise, when the output current limit exceeds its limit, ILIM, the control voltage signal VC is set to the current limit voltage signal VLT.
In general, the current limit voltage VLT is equal to the feedback voltage increased by an offset. The loop created when the output current limit is reached is a positive feedback loop. At the moment the output current reaches the limited current ILIM, the control voltage Vc is set equal to VLT. When the output current is increased by ΔILMT, the feedback voltage decreases by ΔVFB. When the control voltage VC decreases the on-time is reduced. So the power requested by the increased output current is reduced which results in having the output voltage drop linearly. Therefore, the system achieves a substantially constant output current limit with varying output voltage.
As seen in Equation (4), for the given limited output power, the k which represents the on-time and the switching period is inversely proportional to the square of the RMS value of line voltage VIN. The Line Square feed-forward block 302 of the present invention receives the DC input voltage VINDC and squares this signal to generate the squared feed-forward signal Vin2 in order to permit the pulse generator 330 to account for this factor in determining the on-time TON and the switch period TS. The pulse generator receives the squared feed-forward signal VX2 and the control signal VC and modifies the on-time and the switch period of the switch Q1. So it results that the energy to be delivered to the output is identified at low line and high line voltage. Consequently the limited maximum output current ILIM is identified.
That is, the current can then be controlled at a constant level based upon the sensed output voltage, and the square of the input voltage (VIN).
                              t          on          2                =                                            I              o                        ⁢            2            ⁢                          L              M                        ⁢                          T              S                        ⁢                          V              o                                            V            in            2                                              (        4        )            
From this relationship, a feed-forward Vin2 signal is used in the PWM pulse generation circuit, together with a measured VSENSE voltage that is a scaled representation of the output voltage, derived from the primary side auxiliary winding of the transformer. The operation of this circuit is explained in further detail within the Shteynberg patent. The feed-forward squared VIN can be accomplished by any number of methods, both analog or digital by those skilled in the art of design.
This method measures the output voltage by the use of an analog-to-digital circuit (ADC) 304 connected to the VSENSE line of the controller. The current accuracy is influenced by the external components and the turns ratio between the secondary winding and the auxiliary winding on the primary side. The multiplication of the VIN signal by itself is yet another complexity to the implementation. The referenced patent states that the k value which represents the on-time and the switching period is inversely proportional to the square of the RMS value of the line voltage VIN.
What is needed is a primary side sensing current control system and a method that (1) controls the current limit such that it is maintained within a small range of any acceptable input voltage, e.g., 90 to 264 Volts RMS, and (2) causes the output voltage of the PWM controller to drop as the output load increases when the full current is reached, and (3) uses the instantaneous value of the VIN to influence a cycle by cycle calculation of the on-time and off-time, thus allowing removal of the line frequency components from the output by compensating for them on a cycle by cycle basis in the time calculations.